evaluation of grain yield, gas exchange and source and sink limitation in sunflower under drought stress at different levels of defoliation

in order to investigate of the effects of water stress on seed yield and gas exchange and to evaluate the relationship between source and sink in sunflower azargol variety, a split-plot factorial field experiment was conducted in 2008. in this experiment, water stress (no irrigation from 30 days after sowing till ripening) and irrigation (irrigation at -3 bar soil water potential) were considered as main factors and leave removal at five levels (0% as control, 25%, 50%, 75%, and 100% defoliation) as sub factors in flowering period (r5) and end of seed setting (r7). for sink limitation, after pollination, one half of seeds in the head were removed. the results showed that defoliation and drought stress reduced grain yield through biomass, number of filled seed and harvest index reduction. drought stress showed no significant effects on seed oil content, while decreased protein content. results indicated that water stress decreased photosynthesis rate, stomatal conductance and increased water use efficiency, but defoliation increased photosynthesis rate. however it can be concluded that at water stress conditions, both source and sink restricted grain yield whereas at irrigation conditions, the sink limitation determined final grain yield. the reduction of seed oil and increasing source limitation under drought stress conditions showed that use of varieties with lower source limitation associated with increased seed oil content. under irrigated conditions, varieties with high number of grains per head can eliminate sink limitation and lead to improved grain yield.